The present invention relates to multi-pipe flexible flowlines and especially to improvements for preserving certain secondary pipes from corrosion.
These flexible flowlines are intended for the subsea transport of hydrocarbons and comprise in particular a central flexible element, or main pipe, in which said hydrocarbon can flow, and peripheral secondary pipes, the whole assembly being housed inside a flexible external sheath. An annular space between the central flexible element and the external sheath is designed to house the peripheral secondary pipes. These peripheral pipes, made of alloy steel or passivatable metal, extend longitudinally in the annular space, either helically around the central flexible element or sinusoidally around a generatrix of the flowline, more commonly called S/Z, so as to allow these flowlines to bend.
These pipes are designed to transport fluids, for example pressurized gas, between the two ends of a subsea flowline, for example one located at the surface of a platform and the other located near the seabed. In general, this pressurized gas is conveyed from the platform to the seabed in order to be injected into the wellhead so as to make it easier for crude to rise (called “gas lift”).
However, the peripheral pipes are liable to be degraded by corrosion, the more so as seawater, which may contain oxygen, generally circulates within the annular space. Consequently owing to the pressure that is exerted on said pipes, the peripheral pipes may over time become perforated. To overcome this, it has already been imagined to electrochemically preserve the peripheral steel pipes by cathodic protection, by electrically coupling them to sacrificial anodes of the zinc or aluminum type.
The reader may in particular refer to document FR 2 752 908 which describes such a protection device.
However, this type of protection can prove to be insufficient owing to the very high confinement of this annulus and/or can result in the production of hydrogen, which is liable to cause serious embrittlement, called hydrogen embrittlement, in particular in the case of the peripheral pipes made of high-grade stainless steel.